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I. INTRODUCTION

IN order to measure the mobility of grain boundaries during recrystallization, it is imperative to quantify accurately the driving force for migration in the functional relationship V  M  P. It has long since been understood that the driving force for migration of boundaries during the process of recrystallization is the stored energy of cold work. In recent publications, stored energy has been measured as a driving force for recrystallization (in units of stress) in two primary ways: either directly via differential scanning calorimetry (DSC)[1,2,3] or indirectly with orientation imaging microscopy (OIM)[4–9] where stored energy is derived from some measure of orientation gradients. Though both methods have proved useful, measurements of the geometrically necessary dislocation content do not take into account all dislocations[9,10] that might contribute to the overall driving pressure, and, specifically, cannot measure the statistically stored dislocation density because of limitations in accuracy of orientation measurement and spatial resolution inherently not possible by OIM. Calorimetry[11,12] detects an energy change upon reaction, enthalpy, and so includes all the free, or statistically stored, and geometrically necessary dislocations that are eliminated during annealing. Orientation imaging microscopy, however, does have the capability of determining subgrain size and orientation within the limitations of the technique. Typical spatial and angular resolutions in automated electron backscattered diffraction (EBSD) systems are 0.1 m and 0.5 deg, MITRA TAHERI, formerly with the Department of Materials Science and Engineering, Carnegie Mellon University, is Postdoctoral Fellow, United States Naval Research Laboratory. HASSO WEILAND, Scientist, is with the Alcoa Technical Center, Alcoa Center, PA 15069. ANTHONY ROLLETT, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: mitra.taheri@ gmail.com This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A

respectively.[4,5] This article attempts to quantify the stored energy values not only with these two methods, but also with transmission electron microscopy (TEM) for a more precise characterization of subgrain size. Vicker’s microhardness was used to check the accuracy of the calorimetry results. Alloy 1050 was chosen for this study specifically for its low solute content. This, coupled with the high stacking fault energy of aluminum, and a moderately high homologous temperature for cold working, promotes the formation of subgrains in a single-phase microstructure. These characteristics are nearly optimum for the comparison of methods undertaken here.

II. EXPERIMENTAL METHODS A. Differential Scanning Calorimet